How to build Multipass images with Packer

Packer is a utility that lets you (re)build images to run in a variety of environments. Multipass can run those images too, provided some requirements are met (namely, the image has to boot on the hypervisor in use, and cloud-init needs to be available).

Contents:

The basics
Customizing the image
Explaining the process
- cloud-init
- Cleanup
Next steps

The basics

The easiest way is to start from an existing Ubuntu Cloud Image, and the base project setup follows (you can click on the filenames to see their contents, meta-data is empty on purpose):


├── cloud-data
│   ├── meta-data
│   └── user-data
└── template.json

1 directory, 3 files

You will need to install QEMU (e.g. sudo apt install qemu) and from there you can run:


$ packer build template.json
$ multipass launch file://$PWD/output-qemu/packer-qemu --disk 5G
…
Launched: tolerant-hammerhead
$ multipass shell tolerant-hammerhead
…
ubuntu@tolerant-hammerhead ~:$

Customizing the image

Now the above works for you, delete the test instance with multipass delete --purge tolerant-hammerhead and edit the following section in the template.json file:

        {
            "type": "shell",
            "inline": ["echo Your steps go here."]
        },

Anything you do here will be reflected in the resulting image. You can install packages, configure services, anything you can do on a running system. You’ll need sudo (passwordless) for anything requiring admin privileges, or you could add this to this provisioner to run the whole script privileged:

            "execute_command": "sudo sh -c '{{ .Vars }} {{ .Path }}'",

To make the image useful for others, you’ll need to “generalize” the image so that it then works fine on another machine, likely another network, or even under a different hypervisor. That’s what the last entry in the provisioners list does in template.json.

Explaining the process

In short, Packer starts a virtual machine using the downloaded image and runs all the provisioner steps on the running system.

cloud-init

The Ubuntu cloud images are set up to only allow login via SSH with key-based authentication by default. Packer’s qemu builder only supports password authentication, so we use cloud-init to provision a packer user with a simple password for use only during the image build process. This is achieved by the following section in the JSON template:

            "http_directory": "cloud-data",
            "qemuargs": [
                ["-smbios", "type=1,serial=ds=nocloud-net;instance-id=packer;seedfrom=http://{{ .HTTPIP }}:{{ .HTTPPort }}/"]
            ],

This directs Packer to serve the cloud-data directory over HTTP, and cloud-init to consume it via the NoCloud datasource. Refer to cloud-init’s documentation to understand what the user-data entries mean.

Cleanup

For Multipass to be able to consume this image, cloud-init needs to execute another set of instructions, so it has to be reset. The packer user needs to be removed and some other changes need to be reverted in the image (like SSH password authentication). This happens in the last provisioner in the template, which should be self-explanatory.

Your changes are likely to require some cleanup as well, you can mount qemu images and use the Meld tool to compare the two trees, to see if all the changes are indeed on purpose.

Next steps

Go to Packer’s documentation to learn about the QEMU builder, other provisioners and their configuration as well as everything else that might come in handy. Alternatively, you could extend user-data with other cloud-init directives to provision your image.

Please come to our discourse space and let us know about your images!

Last updated 1 year, 1 day ago.

Help improve this document in the forum.